(a) Field of the Invention
The present invention relates to a method for detecting delaminations in coated materials or layered composites generally referred to hereinafter as "layered structures", and to a device for carrying out said method.
(b) Brief Description of the Prior Art
The detection of delaminations and layer unbonds caused by adhesive bond failure at the coating-to-substrate interfaces of coated materials or layered composites such as Al-epoxy laminates or fiber-epoxy structures is an important industrial problem, especially in the construction and aeronautic industries. Indeed, such delaminations and layer unbonds may propagate quickly under certain circumstances and result in a local failure of the coating. In the aeronautic industry, delamination of the aluminum skin adhesively bonded to the honeycomb core of an aircraft wing structure may for example generate in a fast-growing unbonded area and eventually in a catastrophic failure if the delamination is not detected when its diameter is subcritical, typically in the range of 0.5" to 1".
To solve this industrial problem, several methods have been developed for proceeding to a non-destructive inspection of coatings and thereby detecting delaminations in a fast and reliable manner.
A known method consists in subjecting the layered structure to ultrasonic pulse-echo testing. This method which basically consists in measuring the difference in the resonance frequency of an unbonded layer excited by contact with an ultrasonic transducer, is interesting but rather unpractical because it requires a time-consuming, manual scanning and a very good coupling between the surface and transducer, which coupling generally requires the use of an immersion bath.
Another known method for the early detection of delaminations is based on the detection of an acoustic emission. A major problem with this method is the signal interpretation.
A further known method which is relatively recent and considered as a promising approach in this field, consists in taking holographic exposures of the surface of a coating layer before and after deformation of this layer produced by either vacuum, vibration or surface heating. The superposition of the two holograms shows interference fringes when the unbonded surface layer has lifted under the action of the thermally- or vibration-induced stress. Such an optical holography is interesting because it is a non-contact method which does not require scanning. However, such a method has found very little application up to now outside of research laboratories for the following reasons:
(1) Holography is very sensitive to ambient vibrations.
As the exposure time for the holographic plate is relatively long (of the order of 1 second), the inspected structure must therefore be kept on a vibration-insulated table during its exposure, and this exposure must be carried out in absolute darkness while avoiding excessive air currents such as produced by an air conditioning system.
(2) Holography can only detect surface lifts corresponding typically to a vertical displacement of some micrometers. This requires a substantial amount of heating (typically to 10.degree. or 20.degree. C. above ambient), which in turn produces thermal deformations throughout the whole structure, thereby making fringe interpretation difficult. This heating may also cause damage to the adhesive bond.
(3) Holography is relatively unreliable, since the surface lift appears sometimes right after heating, sometimes during cooling depending on the relative thermal expansion coefficients of the surface layer and of the substrate. Moreover layer buckling is only apparent when the unbonded layer is initially convex.
Other methods and devices, for the detection of delaminations have also been proposed, such as for example, the method and sonomicroscope developed by L. W. Kessler and D. Yuhas and marketed by Sonoscan. According to this method, ultrasonic energy is produced by a piezoelectric transducer and applied to a sample. The ultrasonic waves which are transmitted through the sample, produce "ripples" on the sample surface or on a coverlip placed over this surface. Detection of said ripples which are indicative of the internal microelastic structure of the sample, is made by a sensitive, high resolution scanning laser beam microphone the signals of which are processed to form an acousting micrograph display image on a T.V. monitor.
Another method is disclosed in U.S. Pat. No. 3,462,602 to W. R. Apple and basically consists in heating the surface of a layered structure to be inspected with a heat-lamp or a hot-air gun and observing the appearance of the hot spots on the top of the delaminated areas with an IR camera. This infrared method is interesting but prone to errors because of variations in the surface emissivity.
A more quantitative method is described in U.S. Pat. No. 4,468,136 to Murphy and in U.S. Pat. Nos. 4,513,384; 4,521,118 and 4,522,510 to Rosencwaig, and is based on the propagation of thermal waves within the material to be inspected. According to this method, a modulated electron or laser beam is scanned across the surface of a sample. During this scanning, the beam is absorbed by the sample and causes a periodic heating of its surface at the modulation frequency. This heating is the source of thermal waves that propagate from the heated regions and may be acoustically or optically detected. Any sub-surface defect having thermal characteristics different from its surroundings will of course affect these thermal waves and become visible thereto.
This approach is interesting but is has some intrinsic limitations. One of them is that the light of the modulated laser beam may be scattered by the surface toward the detector generating spurious optical noise. A much more important limitation is related to the slow propagation velocity of thermal waves. In order to analyze the state of the coating-to-substrate interface, the modulation period must be of the order of the thermal propagation time through the coating: EQU t.sub.th .perspectiveto.1/f.sub.m .perspectiveto.l.sup.2 /.alpha.
where f.sub.m is the laser modulation frequency, .alpha. is the thermal diffusivity and l is the coating thickness. For a ceramic coating with l=1 mm and .alpha.=1 mm.sup.2 /second, t.sub.th is equal to about 1 second, which means an observation time of at least 10 second if the measurement is extended over a sufficiently large number of modulation periods. This is unacceptably long if a coated part has to be inspected point-by-point for unbond defects over its whole surface.